Method and apparatus for stabilizing of the glow plasma discharges

ABSTRACT

A method and apparatus for stabilizing glow plasma discharges by suppressing the transition from glow-to-arc includes a perforated dielectric plate having an upper surface and a lower surface and a plurality of holes extending therethrough. The perforated dielectric plate is positioned over the cathode. Each of the holes acts as a separate active current limiting micro-channel that prevents the overall current density from increasing above the threshold for the glow-to-arc transition. This allows for a stable glow discharge to be maintained for a wide range of operating pressures (up to atmospheric pressures) and in a wide range of electric fields include DC and RF fields of varying strength.

RELATED APPLICATION

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 09/381,328, filed Dec. 27, 1999 by Erich Kunhardtand Kurt Becker, entitled Method and Apparatus for Suppression of theGlow-to-Arc Transition in Glow Discharges, which is acontinuation-in-part application of U.S. patent application Ser. No.08/820,013 filed Mar. 18, 1997, now U.S. Pat. No. 5,872,426, issued Feb.16, 1999, the entire disclosures of which are expressly incorporatedherein by reference.

GOVERNMENT RIGHTS

The present invention has been make under a government contract and thegovernment may have certain rights to the subject invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a method and apparatus forstabilizing glow plasma discharges, and more specifically to a cathodeconfiguration having a dielectric with a plurality of recesses forstabilizing glow plasma discharges.

2. Related Art

A “plasma” is a partially ionized gas composed of ions, electrons, andneutral species. This state of matter is produced by high temperaturesor strong electric fields created by constant or pulsed DC current, ACcurrent or time varying (e.g., R.F. or microwave) electromagneticfields. Discharge plasmas are produced when free electrons are energizedby electric fields in a background of neutral atoms/molecules. Theseelectrons cause electron—atom/molecule collisions which transfer energyto the atoms/molecules and form a variety of species which may includephotons, metastables, atomic excited states, free radicals, molecularfragments, monomers, electrons, and ions. The neutral gas becomespartially (or fully) ionized and is able to conduct currents. The plasmaspecies are chemically active and/or can physically modify the surfaceof materials and may therefore serve as the basis of new chemicalcompounds and may be used to modify existing compounds. Dischargeplasmas can also produce useful amounts of optical radiation and cantherefore be used in lighting. There are additionally many other usesfor such plasmas. Glow discharges and arc discharges produce a class ofplasmas known as current-maintained plasmas, since they are maintainedby the passage of current therethrough. Such plasmas conduct onlybecause current is passed therethrough and the conductivity falls offquickly if the source of energy to the charge carriers is removed.

Transition points exist at which the various attributes of the dischargeand discharge plasma change from the characteristics of a glow dischargeto the characteristics of an arc discharge. The characteristics thatdistinguish arc from glow are a high gas temperature and a low cathodefall potential, though it is also possible to have a high gastemperature associated with a high cathode fall and vice versa.

The transition from glow to arc passes through a series of stable orquasi-stable states. However, the final step from abnormal glow to arcis very often an unstable change, since a very large potential drop inthe series resistance would be required to make it stable. If there isno series resistance, the transition may take place very rapidly,without equilibrium being achieved in any intermediate stage. Thistransition becomes more rapid as the pressure of the background neutralgas increases towards atmospheric pressure.

In the past, there have been efforts to stabilize glow plasma dischargesin various ways such as the use of source frequencies over 1 kHz,insertion of a dielectric plate (or plates) between two metal electrodesand by using helium dilution gas. Additionally, other attempts tostabilize the glow plasma discharge include placement of an insulatedplate on the lower electrode, use of a brush-style upper electrode, andthe use of a metal upper plate in combination with an insulating plateon the bottom thereof. However, there are certain drawbacks with theserequirements in that, e.g. helium is expensive and there are physicallimitations based on the structure of the electrodes and the insulatedplates.

Past work in this area include a series of articles by Okazaki, Satiko,et al., starting back in 1989 with the article by Kanazaw, S., et al.,entitled, “Glow Plasma Treatment at Atmospheric Pressure for SurfaceModification and Film Deposition,” Nuclear Instruments and Methods inPhysics Research (1989) Elsevier Science Publishers, B. V.(North-Holland Physics Publishing Division), which disclosed a plasmatreatment at atmospheric pressure to stabilize glow plasma by treatmentin a gas which includes carbon-tetrafluoride (CF₄), using helium as thedilute gas and using an insulating plate on a lower electrode, and usinga brush style electrode for the upper electrode to create a stabledischarge at 3,000 Hz.

Yokoyama, T., et al., “The improvement of the atmospheric-pressure glowplasma method and the deposition of organic films,” Journal of Physics(1990) IOP Publishing, Ltd., discloses an improved atmospheric pressureglow discharge plasma method for treating metallic substrates whereinthe middle plate upper electrode is improved by use of an insulatingplate set on its bottom.

Yokoyama, T. et al., “The mechanism of the stabilization of glow plasmaat atmospheric pressure,” Journal of Physics (1990) IOP Publishing,Ltd., discloses stabilization of a glow discharge of atmosphericpressure by controlling three conditions, namely, the use of a highfrequency source, the use of helium gas for dilution, and the insertionof a dielectric plate between electrodes.

Okazaki, Satiko, et al., “Appearance of stable glow discharge in air,argon, oxygen, and nitrogen at atmospheric pressure using a 50 Hzsource,” Journal of Physics, (1993) IOP Publishing, Ltd., discloses amethod and apparatus for stabilizing glow discharge by making thedischarge occur in the early stages of the Kekez curve, and at a lowerdischarge breakdown voltage, by use of a metal wire mesh electrode.

Kogoma, Masuhiro, et al., “Raising of ozone formation efficiency in ahomogeneous glow discharge plasma at atmospheric pressure,” Journal ofPhysics (1994) IOP Publishing, Ltd., discloses an ozone formationapparatus for increasing the efficiencies of ozone formation by use of ahomogenous glow discharge at atmospheric pressure to create ozoneefficiencies increased to about 10% in air to a maximum of 15% in oxygenover conventional filamentary current discharges in gas. The increase isattributed to better collision efficiency among electrons and moleculesand to a lower increase in temperature than in discharge filaments of asilent electric discharge.

Other work in this area includes U.S. Pat. No. 4,498,551 to Hoag,entitled, “Discharge Electrode for a Gas Discharge Device,” which usespin-shaped electrodes which are effectively cooled in the glass flow andwhich promote a stable glow-discharge.

U.S. Pat. No. 5,387,842 dated Feb. 7, 1995 to Roth, et al., entitled,“Steady-State, Glow Discharge Plasma,” and U.S. Pat. No. 5,414;324 datedMay 9, 1995 to Roth, et al., entitled “One Atmosphere, Uniform GlowDischarge Plasma,” both disclose a steady state glow discharge plasmagenerated between a pair of insulated metal plate electrodes spaced upto five centimeters apart and energized with a RMS potential of 1 to 5KV at 1 to 100 Khz. The space between the electrodes is occupied by anoble gas such as helium, neon, argon, etc., and it may also includeair. The radio frequency amplifier means for generating and maintaininga glow discharge plasma includes an impedance matching network. The arcof electric field is high enough to trap the positive ions of the plasmabetween the electrodes, but not so high that the electrons of the plasmaare also trapped during a half cycle of the RF voltage.

None of these previous efforts disclose all of the benefits of thepresent invention, nor does the prior art teach or suggest all of theelements of the present invention.

OBJECTS AND SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a method andapparatus for stabilizing glow discharge plasmas.

It is another object of the present invention to provide a method andapparatus for suppressing the glow-to-arc transition in glow discharges.

It is an additional object of the present invention to provide a methodand apparatus to stabilize glow discharge plasmas in a constant electricfield.

It is even an additional object of the present invention to provide amethod and apparatus to stabilize glow discharge plasmas in time varyingelectric fields.

It is another object of the invention to provide a cathode configurationfor stabilizing the cathode fall in a glow discharge.

It is an additional object of the present invention to provide a methodand apparatus for suppressing the glow-to-arc transition for a widerange of operating conditions and a wide range of operating pressures.

It is also an object of the present invention to provide a method andapparatus for suppressing the glow-to-arc transition for a wide range ofelectric field strengths.

It is an additional object of the present invention to reduce thecomplexity and costs of plasma processing of materials.

These and other objects are achieved by the method and apparatus of thepresent invention for stabilizing glow plasma discharges by suppressingthe transition from glow-to-arc. A dielectric having an upper surfaceand a lower surface and a plurality of recesses extending partiallytherethrough is positioned over the cathode. Each of the holes acts as aseparate active current limiting micro-channel that prevents the overallcurrent density from increasing above the threshold for the glow-to-arctransition.

BRIEF DESCRIPTION OF THE DRAWINGS

Other important objects and features of the invention will be apparentfrom the following Detailed Description of the Invention when read incontext with the accompanying drawings in which:

FIG. 1 is an exploded perspective view of the perforated dielectriccovering a cathode of a DC embodiment of the present invention.

FIG. 2 is a schematic view of a circuit configuration for use with thepresent invention.

FIG. 3 is a graph of voltage v. current for applied voltage, glowvoltage, and arc voltage in Argon at 40 Torr.

FIG. 4 is a graph of voltage v. current for applied voltage, glowvoltage, and arc voltage in Argon at 20 Torr.

FIGS. 5 a and 5 b are graphs of applied voltage, glow voltage and arcvoltage with and without the perforated dielectric of the presentinvention.

FIG. 6 a is a photograph showing an arc discharge and FIG. 6 b is aphotograph showing a glow discharge.

FIG. 7 is a side plan view of another embodiment of the presentinvention for an RF field wherein perforated dielectrics are positionedover both electrodes.

FIG. 8 is a partial view of a dielectric of an alternate embodimenthaving blind apertures.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method and apparatus forstabilizing plasma glow discharges by suppressing the glow-to-arctransition in DC, RF electric field, pulsed DC, AC current or any otherglow discharges which evolves from the cathode fall region. Referring toFIG. 1, it can been seen that a new cathode configuration has beendeveloped to stabilize the cathode fall and suppress the glow-to-arctransition for a wide range of operating conditions. Accordingly, astable glow discharge can be maintained with the cathode configurationof the present invention for a very wide range of operating pressures(up to atmospheric pressures) and in a wide range of electric fieldstrengths.

Referring to FIG. 1, which shows a DC embodiment, the cathode of thepresent invention, generally indicated at 10, comprises a metal cathode20 (aluminum, stainless steel, etc.), covered with a perforateddielectric 30 positioned to face an upper electrode 40. The perforateddielectric 30 may be retained on the cathode 20 by a collar 35 that fitsover cathode 20 and has an aperture 36 therethrough, or may be formed aspart of a cap or cover for the cathode 20, or may be positioned thereonand held in place thereon in any other manner known in the art.

Importantly, the perforated dielectric can be formed of any desireddielectric type substance such as quartz, silicon nitride, siliconcarbide, etc., even glass. The dielectric is preferably formed of amaterial that can withstand high temperatures. Essentially, a perforateddielectric comprises a sieve of holes of micron dimensions. The centerto center distance of the holes is of the same level of dimension. Holedimensions are critical for particular applications. In trials discussedhereinafter, a dielectric having 10 micron holes with a center to centerdistance between the holes of 12 microns was used. Hole dimensions canvary from 5 to 200 μm for the hole diameter and from between 100 μm to 2mm for the hole length (thickness of the dielectric). Importantly, theratio of the hole diameter to the dielectric thickness is an importantfactor and something that can be controlled depending upon theapplication. One example of such a ratio could be 10 to 1, the holediameter being one-tenth of the thickness of the dielectric.

The perforated dielectric can be made by laser ablation. Blanks fordielectric plates made by Norton International can be used, and functionin a desirable matter (a dielectric having a hole diameter of 10 μm, anda hole length of 0.6 mm). The hole diameter, hole lengths, hole density,and material can be varied to optimize the invention for a particularapplication. Any silicon carbide wafer can be perforated by laserablation to form a perforated dielectric for use in connection with thepresent invention.

Referring to FIG. 2, shows a circuit that has been used to conducttrials of the present invention that will hereinafter be discussed,which can be used with the cathode configuration of the presentinvention to effect a stable DC glow plasma discharge. The circuit isgoverned by equation (1): $\begin{matrix}{{V_{s} = {{I_{1}( {R_{1} + R_{2}} )} = {\frac{1}{R_{1}}( {R_{1} + R_{2}} )V}}}{where}{I_{2} = {4I}}{R_{E} \cong {\frac{R_{3}}{4}( {{for}\quad R_{1}{\operatorname{<<}R_{3}}} )}}{V_{g} = {V_{s} - {I_{g}R_{3}}}}{V_{d} = {V_{s} - {I_{d}R_{3}}}}} & {{Equation}\quad(1)}\end{matrix}$

In this way, by measuring voltage V across resistor R, and current ithrough resistor R₁, we can calculate the voltage and the current acrossthe cathode 10.

The present invention allows DC glow discharges, which have a well knowninstability that limits the operating range, to operate at much higherpressure up to atmospheric pressures. Accordingly, this stabilizationallows for applications in many aspects of material processing,pollution remediation, novel lighting devices, and discharge-enhancedcombustion.

The perforated dielectric covering the metal cathode stabilizes thecathode fall region of the DC discharge by breaking the discharge upinto a large number of separate micro-channels. Each of the holescomprising the perforated dielectric acts as a separate, activecurrent-limiting micro-channel. Particle losses due to wall effects andthe finite volume of each channel place an upper limit on the electricalconductivity of each channel, and therefore place an upper limit on thecurrent density that it can carry. This prevents the current densityfrom increasing above the threshold for the glow-to-arc transition.

Additionally, it should be noted that a dielectric material could bedirectly deposited in a proper geometry directly onto a cathode by avapor deposition or other process to apply the dielectric directly tothe cathode. In this way, the cathode itself becomes an activecurrent-limiting device.

A prototype DC glow discharge apparatus was set up using a parallelplate electrode arrangement in an Argon atmosphere of between 10-100Torr, to illustrate the present invention. At these pressures, thephases of the glow-to-arc transition can be readily shown because thetransition is sufficiently slow. The transition at atmospheric pressuresoccurs very rapidly and is difficult to observe. However, it should bepointed out that the present invention is designed to be used atpressures up to atmospheric pressures. Current voltage characteristicswere recorded for a variety of operating conditions using a standardmetal (Al) cathode. The measured curves show the well-known firsttransition corresponding to the breakdown of gas in the formation of astable glow discharge, followed by a prominent second transitioncharacteristic of the transition from the glow regime to an arc whichcreates a filamentary (high current density) channel. Subsequently, theconventional cathode was replaced by the new cathode design and the samecurrent-voltage curves were recorded. All curves showed only the firsttransition to the stable high-current glow. No indications of thepreviously observed glow-to-arc transition were found under anyoperating conditions. The spatial distribution of the discharge is alsodiffuse.

Referring now to FIG. 3, a graph of voltage vs. current for appliedvoltage (VG), glow voltage (Vg) and arc voltage (Vd) is shown for Argonat 40 Torr. FIG. 4 shows a graph of voltage vs. current for appliedvoltage (VG), glow voltage (Vg), and arc voltage (Vd) in Argon at 20Torr.

FIGS. 5 a and 5 b are graphs of the applied voltage and glow-to-arcvoltage with and without the perforated dielectric of the presentinvention. These figures show the stabilization of the glow plasmadischarge. In a first Stage A, there is no current. In the second StageB, voltage is applied, but current stays at zero. In a third Stage C, aglow discharge is achieved. As seen in FIG. 5 a, the glow quickly goesto arc D, while in FIG. 5 b, the perforated dielectric suppresses theglow-to-arc transition and stabilizes the glow discharge such that thereis no arc.

FIG. 6 a is a photograph showing an arc discharge which creates afilamentary (high current density) channel. FIG. 6 b is a photographshowing a glow discharge characterized by a uniform glow discharge.

FIG. 7 is a side plan view of an RF discharge embodiment of the presentinvention wherein the perforated dielectric is positioned over bothelectrodes. As can be seen electrodes 120 and 140 are both covered by aperforated dielectric 130. Because the current reverses itself in a RFelectric field, the dielectric 130 must be positioned over bothelectrodes 120 and 140, as both electrodes alternately serve ascathodes. By this configuration, the glow discharge can be sustainedunder broader operating conditions. Such a configuration results in afrequency independent and size independent device.

By applying the method and apparatus of the present invention to largevolume plasmas at atmospheric pressures, it is possible to increase theenergy released during combustion of fuels to levels several timeshigher than the Heating Value of the fuel. Efforts in this area in thepast have failed because the distribution of energy is required over asubstantial volume and cannot be concentrated in a small area. Becauseof the glow-to-arc transition, there has been tendency to produce arcsof a very high energy level with the rest of the volume remaining at anormal combustion level. By suppressing the glow-to-arc transition andstabilizing the plasma glow, the method and apparatus of the presentinvention overcomes the limitations of the prior attempts and results inan enhancement of the combustion process resulting in much higher energydensities than could be previously achieved.

Additional applications of the present invention may occur in the fieldof air pollution remediation where stabilization of the glow-to-arctransition may result in real time destruction of constituents of airemissions from manufacturing operations in remediation of soil andground water pollution, in large volumes at high pressures. Bysuppressing the glow-to-arc transition and stabilizing the glowdischarge, the present invention creates large volume plasmas to destroythe polluting vapors at higher efficiencies with reduced cost. Therecould be additional applications relating to the destruction ofcombustion by-products such as NO_(x) and SO_(x) which have heretoforebeen destroyed by pulsed corona and barrier discharges.

The present invention is additionally applicable to the cleaning oflithography sheet surfaces in atmospheric pressures. Additionally, theremay be possible utility for large area surface cleaning at atmosphericpressure for curing polymer films. By being able to operate atatmospheric pressure, a great advantage is achieved over the highprocessing cost required in a vacuum process. Additionally, the presentinvention can be used for pretreatment of semi-conductors, glasses, andpolymers which are to be used for direct metal ion beam processing.

Additionally, an atmospheric pressure glow discharge plasma can be usedto sterilize biologically contaminated surfaces. Current techniques inthis area utilize high temperatures, strong chemicals, and/orultraviolet radiation to sterilize contaminated items. However, thereare problems with these approaches in that the processes are timeintensive and potentially hazardous and result in the formation ofpotentially hazardous by-products. It has been demonstrated thatmaterials exposed to a one-atmosphere pressure glow discharge plasma canbe sterilized of biological contaminants in under one minute.

In another embodiment as shown in FIG. 8, the dielectric 230 includesblind apertures 231. By “blind” what is meant is that the apertureextends only partially through the dielectric, not entirely through.Anotherwords, the dielectric 230 includes a plurality of recesses 231extending partially through the dielectric. The recesses are defined bysidewalls 232 and bottom wall 233. A solid component having a thicknessx is defined between bottom wall 233 of the open portion and bottom 234of the dielectric. Importantly, this solid component serves to regulatethe energy available to the discharge when an alternating current isused. The dielectric portion provides for storage capacitance to thedevice and serves to moderate the electric discharge to create a glowplasma that can be tailored for specific applications. The dielectric230 can be made of any dielectric material known in the art. The size ofthe recesses can vary from very small (on the order of 5 μm) to large(millimeters). The thickness of the solid component can range from a fewmicrons to hundreds of microns.

Having thus described the invention in detail, it is to be understoodthat the foregoing description is not intended to limit the spirit andscope thereof. What is desired to be protected by the Letters Patent isset forth in the appended claims.

1. An apparatus for generating and maintaining a glow plasma dischargecomprising: a pair of electrodes positioned in facing relation having aspace therebetween; a dielectric layer having a plurality of recessespositioned over one of the electrodes, each of the recesses having abottom wall, side walls, and a depth less than the thickness of thedielectric layer; and an electric field generated between theelectrodes.
 2. The apparatus of claim 1, wherein each of the pluralityof recesses limits current density from increasing above a glow-to-arctransition threshold.
 3. The apparatus of claim 1, further comprising asecond dielectric layer positioned over the other of the electrodes. 4.The apparatus of claim 3, wherein the second dielectric layer includes aplurality of recesses, each of the recesses having a bottom wall,sidewalls, and a depth less than the thickness of the second dielectric.5. The apparatus of claim 4, wherein each of the plurality of recesseslimits current density from increasing above a glow-to-arc transitionthreshold.
 6. An apparatus for generating and maintaining a glow plasmadischarge comprising: a pair of electrodes positioned in facing relationhaving a space therebetween; a dielectric having a plurality of recessesplaced over one of the electrodes and partially occupying the space, thedielectric including a solid component between lower ends of therecesses and a bottom side; a second dielectric having a plurality ofrecesses placed over the other of the electrodes; and an electric fieldgenerated between the electrodes.
 7. The apparatus of claim 6, whereinthe second dielectric includes a second solid component between lowerends of the recesses and a bottom side.
 8. The apparatus of claim 6,further comprising a retaining collar retaining the dielectric to one ofthe electrodes.
 9. The apparatus of claim 8, further comprising a secondretaining collar for retaining the second dielectric to the other of theelectrodes.
 10. A method of stabilizing glow-to-arc transition for adischarge plasma comprising the steps of: positioning electrodes in afacing relation; providing a dielectric having a plurality of recesses;covering one of the electrodes with the dielectric; and covering theother of the electrodes with a second dielectric including a pluralityof recesses.
 11. The method of claim 10, wherein the step of providingthe dielectric comprises providing a dielectric having a solid componentbetween lower ends of the recesses and a bottom side.
 12. The method ofclaim 10, wherein the step of covering the other of the electrodescomprises covering the other of the electrodes with a second dielectrichaving a solid component between lower ends of the recesses and a bottomside.
 13. The method of claim 10, further comprising retaining thedielectric to one of the electrodes with a first retaining collar. 14.The method of claim 13, further comprising retaining the seconddielectric to the other of the electrodes with a second retainingcollar.
 15. An apparatus for generating and maintaining a glow plasmadischarge comprising: a pair of electrodes positioned in facing relationhaving a space therebetween; a dielectric having a plurality of recessespositioned over one of the electrodes and partially occupying the spacefor limiting current density from increasing above a glow-to-arctransition threshold, the dielectric including a solid component betweenlower ends of the recesses and a bottom side; a second dielectric havinga plurality of recesses placed over the other of the electrodes; and anelectric field generated between the electrodes.
 16. The apparatus ofclaim 15, wherein the second dielectric includes a second solidcomponent between lower ends of the recesses and a bottom side.
 17. Theapparatus of claim 16, further comprising a retaining collar forretaining the dielectric to one of the electrodes.
 18. The apparatus ofclaim 17, further comprising a second retaining collar for retaining thesecond dielectric to the other of the electrodes.
 19. The apparatus ofclaim 15, wherein the second dielectric limits current density fromincreasing above a glow-to-arc transition threshold.
 20. A method ofstabilizing glow-to-arc transition for a discharge plasma comprising thesteps of: positioning electrodes in a facing relation; providing adielectric having a plurality recesses positioned over the electrode forlimiting current density from increasing above a glow-to-arc transitionthreshold; covering one of the electrodes with the dielectric; andcovering the other of the electrodes with a second dielectric includinga plurality of recesses.
 21. The method of claim 20, wherein the step ofproviding the dielectric comprises providing a dielectric having a solidcomponent between lower ends of the recesses and a bottom side.
 22. Themethod of claim 20, wherein the step of covering the other of theelectrodes comprises covering the other of the electrodes with a seconddielectric having a solid component between lower ends of the recessesand a bottom side.
 23. The method of claim 20, further comprisingretaining the dielectric to one of the electrodes with a first retainingcollar.
 24. The method of claim 23, further comprising retaining thesecond dielectric to the other of the electrodes with a second retainingcollar.
 25. The method of claim 20, wherein the step of covering theother of the electrodes comprises covering the other of the electrodeswith a second dielectric including plurality of recesses for limitingcurrent density from increasing above a glow-to-arc transitionthreshold.